The present invention is an RF excited gas arc lamp, such as a krypton arc lamp, capable of producing light of sufficient intensity (watts per cm.sup.2 ) to pump a laser, such as Nd:YAG and Nd:YA10.sub.3 lasers which are pumped with krypton lamps, and which emit copious light in the near infrared portion of the spectrum.
As discussed in the aforementioned application, a particular object of the invention described therein, and a particular object of the present invention, is to provide an arc lamp structure, particularly adapted for krypton arc lamps, that does not have electrodes and thereby avoids the problems, expense and danger of previously used krypton lamps which have electrodes and problems caused by the electrodes, such as rupturing of the seals around the electrodes and sputtering and evaporation of cathode material onto the walls of the envelope.
The present invention has the objects and advantages set forth in the foregoing application and in addition has as its own principal object to provide an improved lamp structure and mounting arrangement relative to the laser rod to be pumped which maximizes the effective surface area of the lamp for laser pumping while minimizing the overall size of the lamp and the assembly for mounting the lamp and laser rod in a laser unit. For example, for a laser unit of a given overall size, a unit embodying the present invention can be adapted to produce an output on the order of more than four times the output from a laser unit of the same overall size constructed in accordance with the annular lamp construction (illustrated by FIG. 1 herein) disclosed in the aforementioned application.
Arc lamps of the type under consideration, and as disclosed by the aforementioned parent application consist essentially of an envelope, suitably of fused silica containing an inert gas, such as krypton, xenon and argon, which is excited by RF voltage applied across a coil around or adjacent to the envelope.
As disclosed in the aforementioned parent application RF voltage (defined for present purposed as voltage having a frequency in the range of from 1.0 to 100 megacycles per second) can effectively be coupled into gas in a lamp envelope and maintain a plasma that provides a sufficiently intense and stable light output to pump a laser if there is provided an appropriate combination of the pressure of the gas, geometry of the envelope and power supply circuitry.
The geometry of an arc lamp envelope is generally expressed in terms of characteristic length, , which is a function of the length and of the lateral dimensions of the envelope and which refers to the average length or distance ions and electrons of the gas atoms travel from the points of excitation to points of impact with a wall of the envelope. In the context of the invention disclosed in the parent application, it is generally desired to have the characteristic length of the envelope and hence its surface areas as small as possible in order to maximize the intensity of light output per unit area of the envelope.
The impedance Z, of the gas plasma is approximately proportional to the mth power of the pressure P divided by the characteristic length , Z.varies..sup.P.spsp.m / , where m is a factor representing the dependence of impedance on pressure for the specific gas. The value of m is close to one; its established value for particular inert gases can be found in tables of impedance dependence on pressure in current reference texts on inert gases for arc lamps. The impedance is critical in the sense that if it is too high, it would be commercially impractical to provide an impedance matching circuit capable of coupling in sufficient RF power to start or maintain the plasma, and the power losses in the coupling circuit would be impractically high.
If the characteristic length is too large, the lamp would be impractical for laser pumping because a major portion of the emitted light could not practically be directed into the laser rod and would be lost; also the intensity of light output per unit of lamp envelope surface area would be reduced. The pressure is critical in that if it is too low, the amount of light radiated is insufficient to provide the required intensity of light; if the pressure is too high, the plasma impedance increases, which increases the difficulty of coupling power in.
The value of the characteristic length, , is determined by the particular configuration of the envelope and by the dimensions of that configuration. These are separate formulae for determining the characteristic lengths of envelopes of different geometric configurations, e.g. spheres, cylinders, annulae, and square sided configurations, the weights given the lateral dimensions, i.e., radii, width, or height, and the weights given the lengths in these formulae vary in accordance with the particular configuration being considered. For example, the general definitions of the characteristic lengths of several representative lamp envelope configurations disclosed and described in the parent application are as follows:
(1) For a cylindrical envelope of length L and radius R, ##EQU1##
(2) For a spherical envelope of radius R, ##EQU2##
(3) For an annular envelope of length L, outer radius R.sub..degree. and inner radius R.sub.i, ##EQU3##
The critical lamp envelope geometry for constructing lamps of this invention is geometry which will have a characteristic length within a particular range. As indicated by the formulae above, the one most significant dimension in determining the characteristic length of an envelope of any configuration is the width of the envelope between its closest walls, and figures for this width and for the length of the envelope (i.e., the largest dimension) will, if inserted in the characteristic length formula for the geometry involved (omitting values for any additional dimensions referred to) give a characteristic length value which is a sufficiently close approximation of the characteristic length to construct lamp envelopes having the requisite characteristics for providing lamps of the invention disclosed in the parent application. Accordingly the critical dimensions of lamp envelopes are the length and the "effective width" which is defined in the parent application as the width of the envelope between its closest walls; in the annular envelope disclosed in the parent application the effective width is the width of the annulus (i.e., which is difference between the inner and outer radii).
The present application discloses that an RF excited inert gas arc lamp which will produce sufficiently intense light to pump a laser and in which impedance matching and sufficient input power are supplied by a fairly simple circuit and a single voltage source is provided by a lamp in which the pressure of the gas, in cold condition before a plasma is generated therein, is in the range of from about 0.5 to about 20 atmospheres and the effective width is in the range of from about 1 to about 30 mm., while its length is not more than about 150 mm.
The power supply circuit for practical operation of the lamp is adjustable to resonance and to match the impedance of plasma in the gas, and may be provided by one of a number of different circuits, for example, a single voltage source applied through a T network or a parallel resonant circuit, or multiple voltage sources, each with a different range of impedances, which may be alternatively applied for substantially matching successively the cold gas impedance and then the impedance of the plasma. For simplicity and economy it would be desirable to be able to utilize a single voltage source and in practice a single voltage source is effectively applied for start up and for maintaining a plasma by a circuit consisting of two variable capacitors respectively in series and in parallel between the RF voltage supply and the coil. In operation, a plasma is ignited in the gas by any suitable means. A simple method is to momentarily energize a Tesla coil whose probe is placed close to the exterior of the lamp envelope. After a plasma is ignited in the gas, a stabilized high current, low voltage for maintaining the discharge is provided by matching the impedance of the power supply circuit to the impedance of the discharging plasma.
The lamp envelope is made a size compatible with the size of the laser rod and is mounted adjacent to the laser rod for as much of the lamp output as possible to impinge on the rod. In operation the lamp generates a great deal of heat which might damage the laser rod, therefore, the usual lamp and laser combination will ordinarily include means for cooling the rod, such as by mounting the lamp envelope and laser rod in a chamber through which water or other light transmissive coolant is circulated.
In addition, in order to have the maximum possible amount of light from the lamp directed into the laser rod, a reflector is placed to direct back toward the rod that portion of light from the lamp which does not fall directly on the rod and which would otherwise be lost. The reflector is preferably a diffuse reflector unless the lamp and rod mounting configuration is such that the reflector can be arranged to focus reflected light onto the laser rod.